Radio frequency filter



June 27, 1939. J. w. CONKLIN RADIO FREQUENCY FILTER Filed April 19, 1937 INVENTOR BY JIM/ff W [pf/KN ATTORNEY Patented June 27, 1939 PATENT OFFICE RADIO FREQUENCY FILTER James W. C'onklin, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application 18 Claims.

This invention relates to a new and novel method of radio frequency filters and is particularly adapted to the lowpass type. The

. ;unique features of my invention are particularly adapted to multi-section filters and provide inherent shielding andmechanical support for the various filter elements. 1

In radio transmittingand receiving apparatus,

wit is frequently desired to filter. the power supply and other external leads to individual unitsfto suppress radio frequency currents from flowing in the leads and from causing undesired radiations or reactions with other equipment and par- .ticularly such equipment'as is operated from a common power supply. In prior art practice, filters for this purpose varied from simple bypass condensers to a multi-section known as the pi or L section networks, which comprised generally a series inductance and shunt capacity. At the high frequencies and particularly in. the so-called ultra-high frequency rangeradio frequency filtering becomes more difficult for several reasons,

namely, that undesired reactances developed inthe interconnecting leads become large and prevent the proper filter action and effective ground circuits are nearly impossible to attain, and

therefore each filter section must be well shielded to prevent the occurrence of stray input to the output coupling.

In the filter of my invention, these difficulties are overcome by a novel arrangement which reduces undesired common reactances and stray couplings to a minimum. In addition, a convenient mechanical assembly is obtained which is particularly well suited to supply lead filtering and the filtering of interconnections between units and various input and output stages. The type of filter of this invention is also well adapted to a resistance coupling network.

This invention will best be understood by referring to the accompanying drawing, in which:

Fig, 1 shows a schematic circuit diagram of a simple pi section filter.

Fig. 2 is an elevation partly in section of a multi-section filter; and

Fig. 3 is an end view of Fig. 2; and Fig. 4 is a perspective view of a modification in which several filter units are combined within a common outer shield.

Referring now in detail to the simple pi section network, shown in Fig. 1, it will be noted that one or more sections are individually shielded from each other by a shield S which also serves as the ground conductor and is. connected at any con-v venient point to ground G. Each filter section April 19, 1937, Serial No. 137,693 I is composed of input and output shunt capacities C1, C2, C3 and C4, connected by series inductances L1 and L1. The constants and characteristics of such a filter are readily determined by means of calculations well known in, the prior art. In 5 practice, however, it is necessary to take into consideration the small unavoidable stray capacities C5 and C6 shunting the inductances. These stray capacities are shown by dash lines on the drawing. In addition to the above mentioned 10 capacities, there occurs other stray capacities such as those indicated as C7, C8 and C9, also shown by dash lines. These capacities are inherent in the construction and occur between the shield and the inductance leads. 15

Fig. 2 and Fig. 3 show the details of a filter employing the principles of my invention. Each individual section of the filter appears as a built up spool located within a metallic outer cover or casing I, the ends of the casing I being provided 20 with a metallic disc or end plate 2 and form the outer condenser electrode C1 of Fig. 1. The individual filter sections are sub-divided by similar. metallic discs or washers 2A, which form the outer electrodes C2, G3, etc., depending upon. the 25 number of filter sections. Washers 2A are secured to casing or cover I by a. plurality of screws 2B. Adjacent the metallic discs 2 and 2A are located the metal flanges 3 which are of the proper size to form the other plates or electrodes of 3'0 the condensers C1, C2, C3, C4, the area being sufficient to give the desired electrical capacity in combination with a suitable dielectric separator I3, of proper dielectric or insulating material, such as mica isolantite, or the like. The indi- 35 vidual series inductance coils ll of the filter are wound upon a spindle 6, which is made of suitable insulating material and form the equivalent series inductances L1 and L2 of Fig. 1. The ends of the spindle 6 are threaded to receive terminal 40' studs 12, which also serve to hold the end ground plates 2 in combination with insulating bushings 4 and 5, which form the dielectric for capacities C7 and C9 of Fig. 1. The electrodes for capacities C1 and C9 are the studs ill and the face of the aperture in plates 2. Adjacent inductance sections are mechanically held together by threaded studs I2 and insulated by insulating bushings forming the dielectric for the capacities Cs. 50

The specific construction shown by Figs. 2 and 3 is intended for a low pass filter cutting off at about twenty-five megacycles. A number of va a o of thi q mqiimisr m d lyep a cut, for example, in a preferred construction. 55'

mica sheets 2 I.

Spacers 2A may be provided with a spring contact arrangement which simplifies the construction generally by the omission of screws 2B, and forms an assembly which permits easy removal of the elements for immediate repair and inspection. The shape of the coils and spindles may be varied to advantage, depending upon the inductance desired. This particular filter unit is intended to be mounted by stud l0, using a suitable insulating bushing through the enclosing shield of an oscillator. An alternative arrangement would be to provide a suitable flange member on shield I for fastening against a receiver shield wall or transmitter panel. In all cases, the shield I is mounted and forms a circuit continuity of any shielding of other apparatus through which it may project. The effectiveness of the shielding and the minimizing of stray coupling reactances attained by this construction will be readily apparent to those skilled in the art. For example, Fig. 4 shows a variation in which several filter units are combined in a common outer shield, thus reducing the number of parts slightly where several filter leads are to enter a radio frequency enclosure. In this modification, a central supporting rod 20 is used for spacing the sections in place of the coil spindles 6, shown in Figs. 1 and 2. The inner electrodes or condenser plates may be formed of a circular or any other shaped disc 21, whenever it is desired to obtain a maximum plate area. There will be some intercoupling between the inductances of the separate filter circuits in this type of filter, but such is not usually objectionable. This modification of filter still retains the advantages of shielding by means of outer casing 22, and metallic end plates 23 and 24. The inductance winding 25 is self supported by being stretched between terminal studs 26, the dielectric to increase the capacity between the condenser plates 23, 24 and 21 being provided by Rod 20, with screws 28, binds the entire unit together, terminal studs 26 being insulated by bushings 29 of insulating material. ihis type of filter still retains the advantages of shielding and low undesired common reactance between the filter sections. Four circuit twosection filters of the type shown in Fig. 4 may be used for filtering the supply leads of two onemeter oscillators which are adapted for a beating oscillator of long line frequency control, whereas the single circuit four-section units shown in Figs. 1, 2 and 3 may be employed for use in filtering six-meter oscillators supplying leads to a high frequency oscillator for television apparatus.

This invention is not to be limited to the devices shown, except such limitations as are clearly imposed in the appended claims.

What is claimed is:

1. A radio frequency filter comprising a continuous metallic shield having metallic end members, said shield being connected to ground, an inductance coil within said shield, a metallic plate located at each end of said coil, said metallic plate serving as one electrode of a condenser for said filter, the other electrode of said condenser being provided by at least one of said metallic end members, and a solid dielectric arranged to increase the capacity of the condenser, said dielectric being interposed between the metallic plates and the metallic end members.

2. A radio frequency filter comprising a continuous metallic shield having. metallic end members, said shield being connected to ground, a plurality of inductance coils within said shield, a metallic plate located at each end of said coils,

at least one of said metallic plates serving as an electrode of a condenser for said filter, the other electrode of said condenser being provided by at least one of said metallic end members, and a solid dielectric arranged to increase the capacity of the condenser, said dielectric being interposed between the metallic plates and the metallic end members.

3. A radio frequency filter comprising a metallic shield having metallic end members, a plurality of series connected inductance coils within said shield, each of said inductance coils being mechanically separated by a metallic plate located at each end of said coils, said metallic plates serving as electrodes of a plurality of shunt connected condenser units for said filter, the other electrodes of said condensers being provided by said metallic members located between said inductance coils.

4. A radio frequency filter comprising a metallic shield having metallic end members, an inductance coil within said shield forming a series inductance for said filter, a plurality of shunt connected condenser units each of which are connected in series with said inductance coil and said metallic shield, a metallic plate located at each end of said coil and serving as one electrode of said condensers, the other electrode of said condensers being provided by at least one of said metallic end members.

5. A radio frequency filter comprising a metallic shield having metallic end members, said shield serving as a ground conductor for said filter, an inductance coil concentrically arranged within said shield forming a series inductance for said filter, a metallic plate located at each end of said coil, said metallic plate serving as one electrode of a shunt connected condenser for said filter, the other electrode of said condenser being provided by at least one of said metallic end members.

6. A radio frequency filter comprising a metallic shield having metallic end members, said shield serving as a ground conductor for said filter, a plurality of series connected inductance coils concentrically arranged within said shield, a metallic plate located at each end of said coils, said metallic plate serving as one electrode of a shunt connected condenser for said filter, the other electrode of said condenser being provided by at least one of said metallic end members.

'7. A radio frequency filter comprising a metallic shield having metallic end members, said shield serving as a ground conductor for said filter, a central metallic member interposed between said metallic end members, a plurality of inductance coils located within said shield and supported between said metallic end members and said central metallic member, said inductance coils serving as series inductances for said filter, and a metallic plate located at each end of said coils and servng as one electrode of a shunt connected condenser for said filter, the other electrode of said condenser being provided by said metallic end members and said central member.

8. A radio frequency filter comprising a metallic shield having metallic ends, said shield serving as a ground conductor of said filter, an inductance coil forming a series inductance for said filter and wound upon an insulating rod, said insulating rod retaining said inductance coil within said shield, a metallic plate located at each end of said coil and serving as one electrode of a shunt connected condenser of said filter, the

other electrode of said condenser being provided by one of said metallic shield ends.

9. Aradio frequency filter comprising a metallic shield having metallic ends, said shield serving as a ground conductor of said filter, a plurality of inductance coils forming series inductances for said filter and wound upon insulating rods for supporting and spacing said inductance coils.

within said shield, a threaded stud fastened to each end of said spacing rods for electrically connecting said coils together in series, a metallic plate located at each end of said coils and serving as one electrode of a shunt connected condenser for said filter, the other electrode of said condenser being provided by one of said metallic end shields.

10. A radio frequency filter comprising a metallic shield having metallic ends, said shield 7 serving as a ground conductor of said filter, a plurality of inductance coils forming series inductances for said filter and located adjacent each other and wound upon insulating rods for supporting said inductance coils withn said shield, a threaded stud fastened to each end of said spacing rods for electrically connecting said coils together in series, a metallic-plate located at each end of said coils and servng as one electrode of a shunt connected condenser for said filter, the other electrode of said condenser being provided by one of said metallic end shields.

11.' A radio frequency filter comprising a me tallic shield having metallic end members, saidshield serving as a ground conductor for said filter, an inductance coil within said shield forming a series inductance for said filter, a metallic plate located at each end of said coil, said metallic plate serving as one electrode of a shunt connected condenser for said filter, an insulating member located between said metallic end plate and said metallic shield. ends, to serve as a dielectric for increasing the'capacity of said condenser.

12. A radio frequency filter comprising a metallic shield having metallic end members, a plurality of inductance coils forming series inductances for said filter and located Within said shield and supported by said metallic end members, a metallic plate located at each end of said coils, said metallic plate serving as one electrode of a shunt connected condenser for said filter, a plurality of insulating members located between said metallic end plate and said metallic shield ends to serve as a dielectric for increasing the capacity of said condenser.

13. A radio frequency filter having in combination an inductance coil and capactive elements, and a solid dielectric arranged to increase the capacity of said capacitive elements, said dielectric being interposed between said capacitive elements, the electrodes of all of said capacitive elements being formed in part from the leads of said inductance coil.

14. A radio frequency filter having in combination a plurality of inductance coils and a plurality of capacitive elements, and a solid dielectric arranged to increase the capacity of said capacitive elements, said dielectric being interposed between said capacitive elements, the electrodes of said capacitive elements being formed in part from lead-in bushings connecting said inductance coils.

15. A radio frequency filter comprising a metallic shield having metallic end'members, an inductance coil within said shield forming an inductance for said filter, a metallic plate located at and connected to each end of said coil, said metallic plate serving as one electrode of a shunt connected condenser for said filter, the other electrode of said condenser being provided by at least one of said metallic end members, and a solid dielectric of mica interposed between said metallic plate and the metallic end members to increase the capacity of said condenser.

16. A radio frequency filter comprising a metallic shield having metallic end members, a plurality of inductance coils Within said shield forming an inductance for said filter, a metallic plate located at and connected to each end of said coils, at least one of said metallic plates serving as an electrode of a shunt connected condenser for said filter, the other electrode of said condenser being provided by at least one of said metallic end members, and a solid dielectric of mica interposed between said metallic plate and the metallic end members to increase the capacity of said condenser.

17. A radio frequency filter comprising a metallic shield having metallic end members, said shield serving as a ground conductor for said filter, an inductance coil within said shield forming a series inductance for said filter, a metallic plate located at each end of said coil, said metallic plate serving as one electrode of a shunt connectedcondenser for said filter, an insulating member of mica located between said metallic end plate and said metallic shield ends to serve as a dielectric for increasing the capacity of said condenser. 

